1. Field of the Invention
The present invention relates to a friction clutch such as an electromagnetic clutch, having a rotor including a drive side clutch plate provided with plastically press deformed depression and a method of manufacturing the rotor.
2. Description of Related Art
A friction clutch such as an electromagnetic clutch has a rotor including a drive side clutch plate and a driven side clutch plate to be attracted toward and frictionally engaged with the drive side clutch plate. Conventionally, in the electromagnetic clutch for transmitting a drive force to a compressor of a vehicle air conditioning system, the rotor including the drive side clutch plate is formed by spinning (refer to JIS B 0122) or cold forging.
Though the rotor can be formed at less manufacturing cost (with higher productivity or inexpensive manufacturing equipment) by spinning than by cold forging, the inventors of the present invention have found on conducting a fatigue test for a purpose of comparing the rotor formed by spinning with the rotor formed by cold forging that the rotor formed by spinning is likely broken down with less load or during shorter time period than the rotor formed by cold forging. Further, an investigation of this reason reveals that a surface of the drive side clutch plate of the rotor formed by spinning is less hardened than that of the rotor formed by cold forging and, as shown in FIG. 7, when bending moment acts on a clutch plate 13 of the rotor due to belt tension applied to an outer cylindrical wall 11 of the rotor, tensile stress occurs on a friction surface 13a of the clutch plate 13. Accordingly, it can be concluded that the rotor having the friction surface 13a whose hardness is lower has weaker resistance against fatigue failure and is more easily broken down by the tensile stress applied thereto.